It has been found that in a 30 mph frontal barrier crash a restraining device such as an air bag must begin restraining the driver by the time the driver has moved forward about 5 inches if it is to prevent injury. Air bag inflating mechanisms require a period of time, such as 30 milliseconds, to effect inflation of the air bag once a signal initiating inflation has been received from a sensor.
It has also been determined that a passenger protective device is needed in accidents in which movement of the vehicle occupant relative to the vehicle reaches a velocity of about 12 mph or greater. Thus, a sensor is required which will determine that the occupant will collide with some portion of the vehicle's interior at a speed of 12 mph or greater and for a 30 mph frontal barrier or equivalent crash initiates air bag inflation a pleriod of time such as 30 milliseconds before the occupant has moved forwarded 5 inches.
It is important that the sensor which initiates operation of the air bag inflation means be responsive to the acceleration of the passenger compartment rather than some other part of the vehicle because this location leads to certain advantages compared with other sensor locations. For example, if a front fender or bumper of the vehicle should collide with a pole or the like which breaks upon or shortly after impact, a sensor mounted on the front fender or bumper could experience a velocity change of 12 mph or greater before the pole breaks, whereas the passenger compartment of the vehicle might experience a negligible velocity change. Under these circumstances, inflation of the air bag is not required and if inflation occurs, it might even contribute to a subsequent accident. It is important, therefore, that the sensor or sensors with which a vehicle is equipped be located in such positions and be of such construction as to be predictive that the passenger compartment will undergo a velocity change necessitating passenger protection.
The mechanical crash sensor designs (as distinct from electronic) that have been made in the largest quantities are the spring mass sensors. The term "spring mass" or "undamped spring means" as used herein means those sensors wherein the motion of the sensing mass is controlled only by the acceleration experienced by the sensor, its translational or rotational inertia and a bias force such as would result from a spring, a magnet or a rolamite-type band. The spring mass sensor is known to function adequately for short duration crash pulses. Such pulses are characteristic of head on crashes or standard barrier impacts. When the vehicle on which the sensor is mounted experiences a deceleration pulse greater than the biasing force on the sensing mass, such as would accompany a crash, the sensing mass moves rapidly towards the forward end of the vehicle, and initiates air bag inflation.
As a point of reference, the maximum acceleration due to application of the vehicle brakes may be considered to be about 0.7 g.
Previously proposed systems have included crash sensors mounted outside the crush zone. Heretofore, all such sensors, have had biases above about 7 g. All of these prior art sensors have been, in fact, level detectors rather than velocity change detectors. In the case of a crash in which a vehicle travelling 30 mph impacts an object causing a constant acceleration of slightly less than than 7 g's, such as might be approximated when a vehicle impacts one of the crash cushioning devices now placed around many rigid roadside structures, the occupant would sustain serious or fatal injuries. This is certainly an accident requiring an air bag and yet no prior art mechanical sensor would have fired. mass. Other situations where prior art mechanical sensors can fail are impacts with snow banks, striking flooded areas of roadways or impacting soft earth.
Furthermore, certain of the prior art sensors when located within the passenger compartment fail to distinguish between an actual crash condition requiring an air bag and conditions not requiring an air bag such as car bottoming, travel over a curb, railroad track or pot hole. Prior art sensors located in the passenger compartment which have high g biases are very sensitive to extremely short pulses normally encountered in such conditions and would cause the air bag to inflate unnecessarily.